Subcutaneous structures and blood vessels under skin are barely visible for naked eyes directly. Without any medical instrument, medical doctors can only rely on the external outline of human body and their anatomic knowledge to recognize and locate subcutaneous structures and blood vessels.
The blood vessels, comprising veins and arteries, are hidden below the epidermis and dermis, and in some cases mixed with the subcutaneous fat or are even behind the bones. Images of the blood vessels under the visible light illumination are therefore extremely faint and barely visible for naked eyes. Before puncture, the doctors often try to make the blood vessels more visible by asking the patients to clench their fist or flapping the skin above the blood vessel, but hindered by patients' age and the thickness of subcutaneous fat and etc., the visibility of subcutaneous blood vessels is still not satisfying in most case. Injections relying on the barely visible images of blood vessels frequently results misalignment of the puncture, causing unnecessary pain in patients and delaying optimal time for medical treatment, even triggering other serious side effect. Apart from blood drawing and injection acted on blood vessels directly, acupuncture and other medical surgery etc. all need the blood vessels to be located accurately, so the blood vessels can be avoided or be treated respectively.
In recent years, a technical approach based on near-infrared (NIR) imaging technology for solving the problem has been proposed. This technical approach is based on the fact that the absorption coefficient of hemoglobin for NIR light from 760 nm and 1000 nm is different from that of other human tissues around the veins, so image contrast is built up. To implement this technical approach, NIR images of veins are acquired in the first step, then the infrared image is digitized and enhanced in contrast and signal to noise ratio by an image processing unit, enhanced image is finally projected back to human skin surface by a visible light projection device. In this technical approach, which has augment reality effect in a broad meaning, the doctors and nurses are able to recognize and locate precisely the subcutaneous blood vessels and conduct various medical treatments and operate in real time.
However, the subcutaneous blood vessels are surrounded by subcutaneous fat and muscular tissues, inevitably causing strong scattering to the infrared image. To add more obstacles, wrinkles, scars and hairs on the skin surface all have strong absorption and scattering effects to attenuate and blur the infrared image. These drawbacks become severe when imaging objects are narrow branches of blood vessels and capillaries. This is simply because that less blood volume and therefore less hemoglobin are in the infrared light path, while the light scatterings from surrounding tissues remain the same, resulting less absorption and faint contrast in infrared image. Under the influence of scattering light, the image contrast of the blood vessels to the surrounding tissues is often observed in the range of 0.01˜0.1. An ordinary method to boost up image contrast is to simply irradiate stronger light onto the image objects. But this method will bring back stronger background scattering of infrared light, which may cause the image device deviated from linear response range or even fully saturated by the flood of light. Adoption of imaging device with higher gain may improve the signal-to-noise ratio to some extent, but will cause the image device saturated earlier or in other words less dynamic range.